1. Technical Field of the Invention
The invention relates to the protection of spacecraft from debris, in the preferred embodiment to the provision of shielding structures and for monitoring space debris impacts on the shielding structures. The invention also relates to the field of de-orbiting devices, in the preferred embodiment of the atmospheric drag type. The invention also relates to debris sweeping apparatus preferably for the removal of debris from the space environment.
2. Description of Related Art
A typical spacecraft in low Earth orbit (LEO) can have a through-life impact-induced probability of failure of up to 5% as a result of interaction with the population of orbital debris larger than 1 mm. Set in the context of the normal spacecraft reliability, this is significant and underlines the need for impact protection. In-flight impact damage data observed on spacecraft such as EURECA, LDEF, and the Hubble Space Telescope reinforces this need.
The Inter-Agency Space Debris Coordination Committee (IADC) has defined space debris as “all man-made objects including fragments and elements thereof, in Earth orbit or re-entering the atmosphere, that are non-functional”. Broadly, there are three sources of debris: launch and mission related objects (LMRO), explosion and collision fragments, and non-fragmentation debris. Currently, LMROs account for much of the on-orbit mass of debris. Most of these objects are observed and tracked by the U.S. Space Surveillance Network (SSN) which maintains a catalog of objects and their associated orbits. The SSN routinely monitors objects larger than 10 cm in low Earth orbit and 1 m at geostationary orbit altitudes (GEO).
According to Dr H. Klinkrad, head of the European Space Agency's Space Debris Office, when classified by object categories, 31.8% of the catalog objects in 2005 were payloads (6% to 7% thereof active satellites), 17.6% were spent rocket upper stages and boost motors, 10.5% were mission-related objects, and the remainder of about 39.9% were debris, mainly from fragmentation events (28.4% caused by upper stages, and 11.5% caused by satellites). When classified according to orbit regimes, 69.2% of the catalog objects were in low-Earth orbits, at altitudes below 2,000 km, 9.3% were in the vicinity of the geostationary ring, 9.7% were on highly eccentric orbits (HEO), including the GEO transfer orbits (GTO), 3.9% were in medium Earth orbits (MEO), between LEO and GEO, and almost 7.8% were outside the GEO region. A small fraction of about 160 objects was injected into Earth escape orbits. Furthermore, in the non-trackable size range, it has been estimated that 10% of 1 mm size debris and 74% of 10 cm size debris are fragments resulting from spacecraft and rocket bodies that have exploded or collided. Another major source of sub-centimeter debris is non-fragmentation in nature. Products released from solid rocket motor firings are the main contributor in this category.
The first recorded collision between a spacecraft and a trackable debris object occurred in 1996 when the Cerise satellite was hit by a fragment from an Ariane rocket stage that had exploded ten years earlier. The collision severed a gravity gradient stabilisation boom causing the satellite to tumble rapidly. More recently, in 2009, the collision between the Iridium 33 satellite and an expired COSMOS 2251 satellite destroyed both satellites and created hundreds of fragments of debris which will remain in orbit for many years, thus adding to the growing population of debris in orbit around the Earth.
Fortunately, such dramatic events are rare as the population of trackable objects in Earth orbit (including debris) is still relatively small (i.e. around 15,000 catalog objects as of July 2009). However, the same is not true of smaller, non-trackable debris. It is estimated that there are tens of millions of millimeter-size pieces of debris orbiting the Earth, therefore the probability of such objects impacting a spacecraft is much higher.
Evidence confirms that spacecraft are routinely hit by small size debris and meteoroids. Examinations of the surfaces of manned spacecraft, such as the Space Shuttle and International Space Station (ISS), and unmanned spacecraft, such as EURECA, LDEF and the Hubble Space Telescope (HST), have revealed a wide variety of impact damage. Craters and holes have been observed on the outer surfaces of these spacecraft, and on their externally mounted equipment.
The consequences of an impact on a spacecraft are dependent on the characteristics of the impactor (such as mass and velocity), the location of the impact, and the design of the spacecraft. Therefore, a wide variety of damage effects can be expected, ranging from negligible to mission-terminating. Meteoroids can impact spacecraft at velocities in the range 1-72 km/s. For orbital debris, the impact velocities in Low Earth Orbit (LEO) can be as high as 16 km/s, however at GEO the relative velocities are less than 1 km/s. At these speeds, it is possible to relate impactor size to damage effect in an approximate fashion. For example, a 1 mm size debris particle can produce a crater or hole as large as 1 cm in diameter, and has sufficient energy to penetrate a typical spacecraft sandwich panel or external equipment. The damage from a 1 cm particle can penetrate deep inside an unmanned spacecraft causing extensive internal damage and potential loss of mission. Even the special purpose multi-layer shields on a manned spacecraft are only just capable of protecting against a 1 cm particle. A 10 cm debris impactor would most likely cause the destruction of a spacecraft.
The impact response of typical spacecraft panels, shields and equipment items, such as electronics boxes, wiring, batteries, solar cells, and propellant tanks are quantified experimentally by their ballistic performance. A significant parameter is the ballistic limit, which is the threshold at which failure occurs when a structure is impacted. For a given impact velocity, it is the minimum size of particle necessary to cause the structure to fail, where failure is usually defined as perforation.
Alternatively, for a given particle size, it is the velocity required for the particle to penetrate a structure.
Broadly, there are two different and distinct approaches that can be considered for reducing the impact vulnerability of spacecraft: 1) modify its architecture in terms of the layout of equipment, or 2) add shielding.
It is known that one approach to enhance protection on unmanned spacecraft is to add layers of shielding mass to honeycomb panels and multi-layer insulation (MLI). It has been demonstrated that this can increase the ballistic limit from about 0.7 mm to over 1 mm. While such improvements are useful, even with this type of enhancement, the probability of failure of a spacecraft can still be quite significant (several percent). At present, multi-layer shields are the most effective type of shielding to protect against particles up to one centimeter in size. One example is the stuffed Whipple shield. An impact shield of the type known in the art is shown in FIG. 1, where a sacrificial impactor disrupting layer 101 is provided above a primary spacing layer 102. This arrangement is held between a cover 104 and a base 105, and fixed to the spacecraft structure 103. The spacing between the disrupting layer and the spacecraft structure, in which the impact cloud may disperse, is represented by the spacing layer 102. Other known devices include that disclosed in WO 00/35753, which describes a multilayered hypervelocity impact shield. These shields are bulky and have a high weight overhead and in general are only used on manned spacecraft.
Spacecraft can be categorized as manned or unmanned. The risk of losing a manned mission warrants the provision of extensive shielding. Currently, there are very few spacecraft that carry astronauts. In Earth orbit, the vast majority of spacecraft are unmanned. They can be categorized according to their function; communication satellites are particularly common, and are generally used to relay radio signals from one point on the Earth's surface to another. Earth observation satellites are also common type of spacecraft and observe the Earth's land, oceans and atmosphere for a variety of reasons, including: scientific research, resource monitoring and management, meteorological (i.e. for weather prediction), geodesy, and reconnaissance and early warning (for military and intelligence purposes). The number of navigational satellites has grown significantly over the past two decades. These enable the determination of location anywhere on the Earth. Another way to classify spacecraft is according to their mass. This is useful because the size of a satellite is directly related to the cost of its launch. Satellite masses range from the very small (less than 0.1 kg) to the very large (more than 1,000 kg). A problem on unmanned spacecraft is to balance the risk of losing the mission to space debris versus the cost, in terms of weight, of providing a high level of shielding.
Since the mass of each subsystem on a spacecraft is carefully controlled any extra mass, for example shielding, must be justified.
Due to their operational interest and unique nature, the GEO and LEO regions are considered as protected regions with regard to space debris to ensure their safe and sustainable use in the future. The GEO protected region, as defined by the IADC is a segment of a spherical shell with the following characteristics:
a) lower altitude=geostationary altitude minus 200 km,
b) upper altitude=geostationary altitude plus 200 km, and
c) latitude sector: 15° South≦latitude≦15° North,
where geostationary altitude is approximately 35,786 km, i.e. the altitude of the geostationary orbit above a spherical Earth with an equatorial radius of 6,378 km. A geostationary orbit is a prograde, zero inclination, zero eccentricity orbit having a period of almost 24 hours. A spacecraft in such an orbit appears to be stationary when viewed from the Earth. The orbit is therefore ideal for certain types of communication satellite and meteorological satellite.
The LEO protected region, as defined by the IADC is a shell that extends from the surface of a spherical Earth with an equatorial radius of 6,378 km up to an altitude of 2,000 km. According to this definition any spacecraft orbiting within this region are said to be in a low earth orbit (LEO).
Medium Earth orbit (MEO) is the region that lies between the above-defined LEO and GEO regions.
A GEO transfer orbit (GTO) is a particular type of highly eccentric (i.e. highly elliptical) orbit (HEO) with an apogee of approximately 37,000 km and a perigee of several hundred kilometers. Spacecraft destined to operate in GEO are initially launched into a GTO.
Two altitudes within LEO are particularly popular for spacecraft operations. These are at approximately 800 km and 1,400 km above the Earth's surface. Unfortunately, these altitudes are also the most heavily populated with orbital debris. Predictions of the long-term growth of the debris population in these valuable regions indicate that routine spacecraft operations may soon no longer be possible because of the collision hazards.
It is against this backdrop that a range of space debris mitigation guidelines have been published. Of particular importance is the need for spacecraft designers and operators to dispose of spacecraft from the LEO region within 25 years of the end of mission, and ideally as quickly as possible.
Removal of spacecraft can be achieved by means including controlled propulsion manoeuvres or by deploying an orbital decay augmentation device. Increasing the surface area to weight ratio of the spacecraft at the end of its lifetime gradually decelerates the spacecraft primarily as a result of its interaction with the Earth's atmosphere (which extends up to several hundred kilometers in altitude). US2009/0218448 describes a satellite air braking wing structure.
De-orbiting devices should ideally bring the spacecraft down out of orbit as quickly as possible to minimize the risk of catastrophic collision with other large objects creating many hazardous debris fragments.
Therefore a large surface area is desirable. However, creating a large surface area presents a problem in terms of weight overhead, which must be kept to a minimum for cost reasons.
A further means of mitigating the risk of space debris is by removing debris from the space environment. It is known to provide spacecraft that are dedicated to the removal of debris, for instance by “sweeping” large panels along an orbital path to absorb or break up debris particles. U.S. Pat. No. 4,991,799 describes an orbital debris sweeper. A disadvantage of these systems is that it is very costly to provide a dedicated sweeping vehicle.
The implementation of measures to improve the survivability of spacecraft against debris impacts is a recommendation of the UNCOPUOS Scientific & Technical Subcommittee's Technical Report on Space Debris published in 1999. One of the most common ways to do this is through enhancements to the spacecraft structure, such as the addition of shielding.